TRAVERSE CITY, Michigan — Two gray wolves recently traversed the frozen Lake Superior surface from Canada to Isle Royale National Park, scientists said Tuesday, but the animals stayed only five days — dashing hopes that ice bridges would induce migrants from the mainland to replenish the island's lagging wolf population.

One wolf was a female that had been fitted with a radio collar last year, enabling biologists to trace her movements. The other was smaller — perhaps an offspring of the female. There is no evidence that either mated during their short time at the island park, said Rolf Peterson, a Michigan Technological University scientist who studies wolves and moose there.

Scientists have long believed wolves first made their way to Isle Royale in the late 1940s by crossing ice that forms during particularly cold winters. They found a plentiful food supply in the moose that inhabit the archipelago in northwestern Lake Superior.

The wolf population has averaged 23 but has fallen drastically in recent years because of inbreeding, disease and a temporary moose shortage, scientists say. Only eight were counted in 2013, the least since the 1950s. Last year's total was nine.

Peterson and his Michigan Tech colleague John Vucetich recently completed their annual wolf and moose census. Peterson declined to discuss the findings Tuesday but said a report would be released in coming weeks.

It was disappointing that the two wanderers didn't remain at the park, said Seth Moore, biology and environment director for the Grand Portage Band of Lake Superior Chippewa in Minnesota, which collared the female. Ice between Isle Royale and the mainland has broken apart and, with temperatures moderating, is unlikely to re-form this winter.

A bridge also took shape during last year's bitter winter, but the only wolf known to cross was a female that left the island for the mainland, where she was shot dead.

Still, Moore said this year's visit further confirmed that ice bridges were the means by which wolves originally populated Isle Royale.

"It's what we always believed, but it's nice to see crystal-clear evidence," he said.

Scientists increasingly suspect other migrants have arrived undetected over the years and diluted the wolves' gene pool, which might explain why they have survived so long, Peterson said.

But human intervention may be the only way to guarantee their long-term survival, he said. The failure of any permanent newcomers to arrive this winter or last shows that migration is unreliable. And if the climate warms as scientists predict, ice bridges likely will become rarer.

Also, the radio-collared female had severe mange and might not have been a good addition to the park's fragile population anyway.

"For wolves to arrive at Isle Royale and successfully breed, that's a taller order than just running around on an ice bridge," Peterson said. "To execute a genetic rescue, you'd get the right sex and absolutely try and clean them of diseases and parasites" before transporting them to the island.

Park Superintendent Phyllis Green ruled out an immediate rescue last April but said staffers would develop a long-range management plan that considers all options.

The two wolves' recent visit "will certainly help inform our decisions as we move forward," Green said Tuesday.